The limulus lysate test is currently the most sensitive assay for the detection of bacterial endotoxin. The sensitivity and specificity of the limulus test for the detection of bacterial endotoxin in blood, cerebral spinal fluid, joint fluid and urine in man have been and are being evaluated to determine the clinical utility of this assay. Compounds other than bacterial endotoxin that can cause a positive limulus reaction are being characterized. A nephelometric method for performing the limulus assay to enhance sensitivity and specificity is being developed.